Biochar altered native soil organic carbon by changing soil aggregate size distribution and native SOC in aggregates...

Soil aggregates play an important function in soil carbon sequestration because larger aggregates have higher soil organic carbon contents. A field experiment was set up in 2009 that included four treatments, i.e., B0, B30, B60, and B90 representing biochar application rates of 0, 30, 60, and 90 t ha^-1, respectively. In 2017, we investigated the soil aggregate distribution, biochar and n-SOC contents in soil and different aggregate sizes using the ignition method, as well as the contribution of wheat and maize residues to n-SOC content in each aggregate by isotopic analysis. The results showed that, relative to B0, the n-SOC content presented an 14.0% decrease in B30, compared with an 18.8% and 8.2% increase in B60 and B90 (p < 0.05), respectively. Furthermore, the decreased n-SOC content in B30 was due to the decreased proportions of < 53 μm and 1000-250 μm aggregates. The increased n-SOC content in B60 was due to the significantly enhanced proportion of 2000-1000 μm and 1000-250 μm aggregates because the n-SOC contents of these two aggregates size classes were not changed by biochar. However, in B90, the increased n-SOC content was ascribed to the enhanced proportions of 2000-1000 μm and < 53 μm aggregates, although the n-SOC content in 2000-1000 μm aggregate was significantly decreased by biochar. Further analysis showed that the decreased n-SOC content in 2000-1000 μm aggregates was associated with decreased wheat-derived n-SOC content. In synthesis, our study showed a long-term effect of biochar on the n-SOC content by mainly changing soil aggregation and native organic carbon derived from wheat residue, and this effect was dependent on the applied amount. The biochar rate of 60 t ha^-1 is recommended for carbon sequestration in terms of the more pronounced negative priming of native SOC, while the feasible combination between other biochars and soils needs further clarification.